Microwave harmonic filter employing cascaded alternate e-plane and h-plane t-junctions

ABSTRACT

A plurality of cascaded alternate E-plane and H-plane Tjunctions pass the fundamental of a microwave signal fed thereto while substantially attenuating harmonics of the fundamental. Each of the junctions includes an input arm responsive to the wave, an output arm at right angles to the input arm and a further arm aligned with the input arm. The further arm includes parallel waveguides for preventing substantial propagation of the fundamental while providing propagation of the harmonics. At the end of each of the parallel waveguides a microwave absorber is provided to substantially attenuate the harmonics.

[111 '9 3,805,193 [451' Apr. 16, 1974 United States Patent Mohr EMPLOYING CASCADED ALTERNATE Primary ExaminerJames W. Lawrence EPLANE D N JUNCTIONS Assistant Examiner-Wm. H. Punter A z A Hans Jacob Mohr, 1509 Fordham g zgf g g Stanley Z Cole Allan M Way, Mountain View, Calif. 94040 Mar. 5, 1973 Appl. N0.: 337,972

[ MICROWAVE HARMONIC FILTER [76] Inventor:

[57] ABSTRACT A plurality of cascaded alternate E-plane and H T-junctions pass the fundamental of a microwa [22] Filed:

-plane ve signal fed thereto while substantially attenuating harmonics of the fundamental. Each of the junctions includes an input arm responsive to the wave, an output arm at right angles to the input arm and a further arm aligned with the input arm. The further arm includes parallel waveguides for preventing substantial propagation of the fundamental while providing propagation of the harmonics. At the end of each of the parallel wave- 62 0 1 1 3563 3 73 3 ,3 1 WH m MW 3 3 91/ d 60 e wa n a C U S m: m n WWW .m "mm 6 a... R Ha Hue U l C d 5nd U.mF 1 1] 2 8 6 555 5 [.lzl.

333/73 W guides a microwave absorber is provided to substantially attenuate the harmonics.

11 Claims, 4 Drawing Figures UNITED STATES PATENTS 2,762,986 9/1956 2,795,763 6/1957 Tillotson 2,961,619 11/1960 Breese et al. Rooney............................

MICROWAVE HARMONIC FILTER EMPLOYING CASCADED ALTERNATE E-PLANE AND II-PLANE T-JUNCTIONS FIELD OF INVENTION The present invention relates generally to microwave harmonic filtering networks and more particularly to a microwave harmonic filtering network wherein cascaded, alternate E-plane and H-plane T-junctions are provided.

BACKGROUND OF THE INVENTION Electromagnetic waves derived from microwave sources include a fundamental component, and usually one or several higher harmonics. To prevent interference with microwave signals at higherfrequencies than the fundamental it is necessary to eliminate the harmonics prior to transmission of the microwave energy. One prior art device for substantially eliminating or attenuating harmonics of microwave energy, while passing the fundamental without substantial loss, is a structure known as a leaky wall filter. Leaky wall filters, however, require a substantial volume and, may, :in order to provide the desired degree of harmonic attenuation insert a substantial amount of attenuation on the fundamental.

In an attempt to obviate the difficulties arising with the use of a leaky wall harmonic microwave'filter, attempts were made' to employ a plurality of cascaded E- plane or H-plane T-junctions, each of which includes structure for passing the fundamental without attenuation and for substantially attenuating harmonics. Such a structure includes an input arm, an output arm at right angles to the input arm, and a further arm aligned with the input arm. The further arm includes a pair of parallel waveguides having a cutoff frequency greater than the fundamental but less than the frequencies of the harmonics. Thereby, the fundamental is coupled from the input arm to the output arm, while the harmonics propagate in the parallel waveguides of the further arm. Microwave absorbers are provided at the ends of the parallel waveguides to absorb the harmonic energy and prevent reflection thereof back to the input and output arms.

In cascading together a number of E-plane T- junction harmonic attenuatorsof this type or a number of H-plane T-junctions of this type, it was found that the attenuation characteristics of harmonic stop bands do not add as is normally expected. In the expected situation, if db of attenuation wereattained in each T- junction and three such junctions were cascaded together, 30 db of attenuation in the stop band should occur. However, experiments indicate that attenuation in the stop band is considerably reduced and that in the example given harmonic attenuation may be as low as db. Apparently, the reduction in attenuation in the stop band for the harmonics occurs because of mode conversions by the filter structure comprising the parallel waveguides in the further arm. The harmonics are apparently converted in the filter structure into a mode thatpropagates readily in the output arm of the next cascaded T-junction. Since there is a relatively short distance between adjacent T-junctions there is substantial coupling of the mode converted harmonics and the theoretical attenuation for the harmonics is not provided.

BRIEF DESCRIPTION OF THE INVENTION In accordance with the present invention, E-plane and I-I-plane harmonic attenuation T-junctions of the prior art type are cascaded so that the E-plane and H- plane T-junctions alternate. This is in contrast with the previously attempted approach wherein all of the harmonic filters are either of the E-plane T-junction type or all are of the I-I-plane T-junction type. Since the E- plane and H-plane T-junctions inherently provide mode conversion, alternating the E-plane and l-l-plane T-junctions prevents propagation of the converted mode of the harmonics by the filter structure. Propagation of the converted harmonic mode is substantially prevented since an E-plane junction generally exhibits greater harmonic attenuation when measured in the TE mode, while an I-I-plane junction generally exhibits greater harmonic attenuation when measured in the TE mode.

1 It is, accordingly, an object of the present invention to provide a new and improved microwave harmonic filter.

Another object of the invention is to provide a new and improved microwave harmonic filter requiring relatively low volume, without substantially attenuating fundamental frequency components.-

A further object of the invention is to provide a new and improved microwave harmonic filter wherein a plurality of T-junction harmonic'filters are cascaded such that the attenuation characteristics of the cascaded filters are substantiallyadditive in terms of db of attenuation.

A further object of the invention is to provide a microwave harmonic filter wherein a plurality of T- junction harmonic filters are cascaded together such that mode conversion of the harmonics within the filter structure does not result in coupling of substantial harmonic energy to the filter output.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of an embodiment ofthe invention utilizing the cascaded combination of an H plane harmonic filter, an E-plane harmonic filter and an I-I-plane harmonic filter; and

FIGS. 2-4 are various views of a harmonic filter in accordance with the invention wherein six attenuating E-plane and I'I-plane Tjunction harmonic filters are cascaded together so that the mode and polarization of the input and output waves are the same.

DETAILED DESCRIPTION OF THE DRAWING Reference is now made to FIG. 1 of the drawing w'hereinthere are illustrated three cascaded harmonic filter T-junctions 11-13. Alternate ones of T-junctions 11-13 are of the I-I-plane and E-plane type, whereby junctions l1 and 13 are 'H-plane T-junctions, while junction '12, which is between junctions 11 and 13, is

an E-plane T-junction. Each of T- junctions 11-13 includes an input arm, an output arm at right angles to the in-put arm and a further, harmonic filtering arm including means for preventing substantial propagation of the fundamental of microwave energy therein and for substantially attenuating harmonics of the fundamental. The arms of couplers 1l-13 are denominated by reference numerals as follows:

Junction Input Arm Output Arm Filtering Arm 11 15 18 21 l2 l6 19 22 13 17 23 Input arm 15 is responsive to a microwave signal having a fundamental and one or more harmonics. The three cascaded filter sections pass the fundamental without substantially attenuating it but attenuate the harmonics such that the harmonic attenuation for all three sections is substantially the sum of the attenuation of one section. Output arms 18 and 19 of T-junctions 11 and 12 are coupled directly to or essentially form input arms 16 and 17 of T-junctions 12 and 13, respectively.

Considering the details of T-junction 11, input arm 15 comprises a rectangular waveguide in which is launched the microwave input signal in the TE, mode, whereby the electric field vector, E, extends between the horizontally extending, parallel long walls 26 and 27 of the waveguide.

Filter arm 21 of T-junction 11, which includes means for preventing propagation of the fundamental and means for attenuating harmonics of the fundamental, comprises a waveguide element having peripheral horizontal and vertical walls that are aligned with and coplanar with corresponding walls of the waveguide of arm 15. Arm 21 comprises a pair of parallel rectangular waveguides 29 and 30 which are formed by a metal septum 32 that extends longitudinally of arm 21 such that the cross-sectional areas of waveguides 29 and 30 are equal. septum 32 extends between and engages the horizontally extending walls of arm 21, from the right end i of arm 21 (as viewed in FIG. 1), and includes a vertically extending left edge approximately aligned with the center of output arm 18 of T-junction 11 to a point approximately aligned with the right edge of the rectangular waveguide forming arm 18. Waveguides 29 and 30 are dimensioned so that the cutoff frequency thereof is greater than the frequency of the fundamental of the microwave signal launched in input arm 15, but less than the harmonic of the fundamental. Thereby, the fundamental does notenter arm 21, while the harmonics are free to propagate in waveguides 29 and 30. The harmonics are substantially attenuated in waveguides 29 and 30 since the waveguides are provided with pyramidal absorbers 33 at their extreme a mode that cannot readily propagate in the output arm 19 of E-plane T T-junction 12.

Output arm 18 of H-plane T-junction 11, input arm 16 of E-plane T-junction l2 and the harmonic filter arm 22 of junction 12 are formed by the same rectangular waveguide 38, having a longitudinal axis at right angles to the longitudinal axis of the waveguide that forms arms 15 and 21. The horizontally extending walls 39 and 40 of waveguide 38 are respectively coplanar with the horizontally extending walls 26 and 27 of the waveguide comprising arms 15 and 21, while the vertically extending walls 41 and 42 of waveguide 28 extend at right angles to the vertically extending walls of the waveguide forming arms 15 and 21.

Attenuation of the harmonic energy is further provided by E-plane T-junction 12 by virtue of a further pair of rectangular waveguides 43 and 44 that are formed by septum 45 in a similar manner to that by which waveguides 29 and 30 are formed in arm 21 of I-l-plane T-junction l1. Microwave absorbers 45 are provided at the ends of waveguides 43 and 44 as described supra with regard to absorbers 33. Septum 45 extends from the ends of waveguides 43 and 44 to anouter edge approximately aligned with the waveguide comprising output arm 19 of E-plane T-junction 12. The outer edge of septum 45 is provided with a taper that extends from the upper face of waveguide 38 from a point approximately in alignment with the left wall of output arm 19 downwardly to a point that is approximately in alignment with the right face of arm 19. While the energy of the fundamental is propagated throughout the device in the TE mode, harmonics can be mode converted by obstacles in the filters, e.g., the septums and absorbers, connected to T-junctions l113. Thereby, there is some propagation in the TE mode in waveguide 38. The TE mode of the harmonics in waveguide 38 is not easily coupled through T- junction 12 to output arm 19 but is easily coupled to filter arm 22, thereby providing increased harmonic attenuation.

Since the construction of T-junction 13 is substantially identical to the construction of T-junction 11, no further description thereof is provided.

Reference is now made to FIGS. 2, 3 and 4 wherein there are illustrated top, side and front views of an embodiment of the invention wherein six cascaded, alternate H-plane and E-plane T-junctions 51-56 are provided. I-I-plane T-junctionsSl, 53 and 55 are respectively illustrated in plan view in FIGS. 2, 3 and 4, while E-plane T-junction 52, 54 and 56 are respectively illustrated in plan view in FIGS. 4, 2 and 3. Each of the H- plane and E-plane junctions is identical to the H-plane and E-plane junctions illustrated in FIG. 1 and thereby includes an input arm, an output arm at right angles to the input arm and a harmonic filtering arm for preventing propagation of the fundamental energy and means for attenuating harmonics of the fundamental. With the six-arm device of FIGS. 2-4 the polarization of the fundamental at the output arm of the last E-plane junction 56 is identical to the polarization of the fundamental supplied to the input arm of the first H-plane coupler 51 and the input and output waveguides are in line.

While there have been described and illustrated several specific embodiments of the invention, it will be clear that variations in the details of the embodiments specifically illustrated and described may be made without departing from the true spirit and scope of the invention as defined in the appended claims.

I claim:

1. A microwave network for passing the fundamental of a microwave signal fed thereto while substantially attenuating harmonics of the fundamental, comprising: cascadedalternate E-plane and l l -plane T-junctions, one of which is responsive to the signa lfeach of said junctions including: an input arm responsive to energy of the signal, an output arm responsive to the energy in the input arm and at right angles to the input arm, and a harmonic filtering arm aligned with the input arm, said filtering arm including means for preventing substantial propagation of the fundamental therein and means for substantially attenuating-the harmonics.

2. The network of claim 1 wherein the means for preventing substantial propagation of the fundamental includes parallel waveguides in the filtering arm, each of said waveguides having a cutoff frequency greater than the fundamental frequency and less than the frequency of harmonics to be attenuated.

3. The network of claim 2 wherein the means for substantially attenuating the harmonics comprises microwave absorbers at ends of the parallel waveguides.

4. The network of claim 3 wherein the parallel waveguides are formed by a metallic septum extending longitudinally in the further arm, said septum extending between parallel walls between which an electric field vector of the hannonic extends.

5. The network of claim 4 wherein each of said septums is tapered in the longitudinal direction for each E-plane junction. 1

6. The network of claim 4 wherein each of said absorbers is pyramidal.

7. The network of claim 1 wherein six of said junctions are cascaded together, said six couplers being arranged so that the polarization of the fundamental derived from the output arm of the last.coupler is the same as the polarization of the fundamental supplied to the input arm of the first coupler.

8. The network of claim 7 wherein the fundamental propagates throughout the network in the TE mode and the filtering arms include means for mode converting harmonics of the fundamental.

9. The network of claim 7 wherein the input arm of the first junction and the output arm of the sixth junction are aligned. I

1 10. The network of claim 1 wherein the fundamental propagates throughout the network in the-TE, mode aligned walls. 

1. A microwave network for passing the fundamental of a microwave signal feed thereto while substantially attenuating harmoincs of the fundamental comprising cascaded alternate Eplane and H-plane T-junctions one of which is responsive to the signal, each of said junctions including: an input arm responsive to energy of the signal, an output arm responsive to the energy in the input arm and at right angles to the input arm, and a harmonic filtering arm aligned with the input arm, said filtering arm including means for preventing substantial propagation of the fundamental therein and means for substantially attenuating the harmonics.
 2. The network of claim 1 wherein the means for preventing substantial propagation of the fundamental includes parallel waveguides in the filtering arm, each of said waveguides having a cutoff frequency greater than the fundamental frequency and less than the frequency of harmonics to be attenuated.
 3. The network of claim 2 wherein the means for substantially attenuating the harmonics comprises microwave absorbers at ends of the parallel waveguides.
 4. The network of claim 3 wherein the parallel waveguides are formed by a metallic septum extending longitudinally in the further arm, said septum extending between parallel walls between which an electric field vector of the harmonic extends.
 5. The network of claim 4 wherein each of said septums is tapered in the longitudinal direction for each E-plane junction.
 6. The network of claim 4 wherein each of saiD absorbers is pyramidal.
 7. The network of claim 1 wherein six of said junctions are cascaded together, said six couplers being arranged so that the polarization of the fundamental derived from the output arm of the last coupler is the same as the polarization of the fundamental supplied to the input arm of the first coupler.
 8. The network of claim 7 wherein the fundamental propagates throughout the network in the TE10 mode and the filtering arms include means for mode converting harmonics of the fundamental.
 9. The network of claim 7 wherein the input arm of the first junction and the output arm of the sixth junction are aligned.
 10. The network of claim 1 wherein the fundamental propagates throughout the network in the TE10 mode and the filtering arms include means for mode converting harmonics of the fundamental.
 11. The network of claim 1 wherein each of said arms comprises a rectangular waveguide, the input and output arms of adjacent junction being waveguides having aligned walls. 